Lignocellulosic material, e.g., wood chips, saw dust and other fibrous material from wood or plant, is refined by mechanical refiners that separate fibers from the fiber network that forms the lignocellulosic material. A typical refiner for processing fibrous material is a disc-type refiner, wherein two refiner plates—which also are referred to as refiner discs—are positioned opposite to each and wherein at least one refiner plate rotates with respect to the other refiner plate. A rotating refiner plate or refiner disc may be referred to as a rotor, while a stationary refiner plate or refiner disc may be referred to as a stator. The rotor and stator plates comprise refiner plate segments, which can be mounted directly on the rotor and stator, respectively, or can be mounted by means of special segment holders. The lignocellulosic material to be refined is fed into a central inlet in at least one of the two refiner plates, and moves therefrom into a refining gap arranged between the two refiner plates. As at least one of the refiner plates rotates, centrifugal forces created by the relative rotation between the two refiner plates move the lignocellulosic material outwards and towards the periphery of the refiner plates. The opposing refiner plates comprise refiner plates segments, which have surfaces that include bars and grooves; and the lignocellulosic material is—in the refining gap provided between crossing bars of the opposing refiner plate segments—separated into fibers by forces created by the crossing bars as the refiner plates rotate in relation to each other. Besides moving in the refining gap between the bars of the opposing and rotating refiner plates, lignocellulosic material may also move outwardly within the grooves provided in a refiner plate segment. When moving within a groove, the lignocellulosic material is not subjected to the aforementioned forces created between crossing bars of opposing and rotating refiner plates, and there is consequently no fiber separation. To remedy this unwanted situation, it is known to provide the grooves with dams, i.e. transverse restrictions, which force the flow of lignocellulosic material out of a groove and into the refining gap between crossing bars between opposing and rotating refiner plates. The U.S. Patent Application No. 20140110511 to Antensteiner discloses refiner plate segments of this type, which include fully dammed grooves or partially dammed grooves. At least some of the dams disclosed in this patent application are full-height dams, which implies that the bottom of the dam is the substantially flat bottom surface of the groove in which the dam is positioned and the top of the dam is at substantially the same height as the bars surrounding and defining this groove.
Although a refiner plate design with dammed grooves works well for its main purpose, i.e. to force the flow of lignocellulosic material out of a groove and into the refining gap between crossing bars of opposing and rotating refiner plates, it is associated with drawbacks. A dam of this type, in particular a full-height dam, is subjected to a considerable wear since the flow of lignocellulosic material in a groove encounters and collides with the dam at high speed and at an almost perpendicular angle. Thus, the dam arrangement constitutes a substantial flow restriction, which creates forces that act against the rotational direction of the refiner plate. Refiners comprising refiner plates with dammed grooves are therefore typically accompanied by a high energy consumption. Since dams force the flow of lignocellulosic material out of a groove and into the refining gap in a rather abrupt and uncontrolled way, the flow distribution is typically also less than optimal in a refiner plate segment with dammed grooves, which, in turn, may lead to a deteriorated, or at least compromised, fiber quality.
An object of the present invention is therefore to provide an improved refiner plate or refiner plate segment, which reduces the wear of the dams and thereby provides a longer useful segment life-time. A further object is to provide an improved refiner plate or refiner plate segment, which reduces the energy consumption in a refiner equipped with such refiner plates. A still further object is to provide an improved refiner plate or refiner plate segment, which provides a better flow distribution and thereby an improved fiber quality.